Laying new underground cables in the Swiss transmission grid only remains possible to a very limited extent. This is the result of the Swissgrid cable study, which shows that the physical properties of underground cables make stable grid operation and the rectification of disturbances much more difficult. Uncontrolled underground cabling has a negative impact on Switzerland’s security of supply. For future grid projects, the possibility of installing an underground cable should therefore be considered from an overall perspective.
Overhead line or underground cable? Partial or full cabling? These issues are often the subject of emotional and controversial debate in line projects for the transmission grid (380 and 220 kilovolts). As an innovative company, Swissgrid is fundamentally open to all technologies and examines both overhead line and underground cable options for every project. The technology is not chosen by Swissgrid, but by the Federal Council – on the recommendation of a support group appointed by the Swiss Federal Office of Energy (SFOE). This process and the isolated assessment of each project without taking a long-term view of the overall system has led to a sharp increase in the number of underground cables in the transmission grid in recent years. 42 kilometres of underground cable have now been installed. Official decisions have already been made to lay a further 250 kilometres.
A cable study to analyse the consequences of the rise in underground cables
The more widespread use of underground cables highlights their effects on the overall system – both in normal operating conditions and in the event of disturbances. Swissgrid has prepared a cable study in order to clarify the situation and to look objectively at the discussion on the choice of technology for line projects. The study simulates the Swiss transmission grid on the basis of scenarios with varying degrees of underground cabling. The study has been validated by the company RTEinternational and reviewed by the Swiss Federal Office of Energy (SFOE) and the Federal Electricity Commission (ElCom). In addition, the results of the study commissioned by the SFOE have been confirmed by the Swiss Federal Institute of Technology (ETH).
The cable study confirms what Swissgrid has been pointing out for some time: an increase in underground cables poses major technical and operational challenges in the extra-high-voltage grid (380 and 220 kilovolts), which could have a negative impact on Switzerland’s security of supply. These challenges are linked to the specific physical properties of underground cables, which increase disproportionately as the voltage rises. To understand these phenomena, it is necessary to delve deep into the world of electrical engineering.
Voltage maintenance: more foam, less beer – and higher costs
A constant voltage is essential to ensure the reliable operation of the Swiss transmission grid. To maintain the voltage, power plants have to produce or absorb reactive power with their generators. Like foam that fills a glass and leaves less space for beer, reactive power reduces the active power of a power line – without doing any useful work.
This is where underground cables come into play. They generate more reactive power than overhead lines on account of their compact design: their conductors are much closer together than in overhead lines, and they are encased in a thick insulating sheath. This increases their capacity – in simple terms, their ability to absorb and return electrical charges – and hence also their reactive power. This has consequences. If there is too much reactive power in the grid, there is a risk of damage due to overvoltages. Even now, situations already occur in which power plants are unable to absorb enough reactive power to keep the voltage within the reliable limits in all regions.
This situation is exacerbated by the increase in underground cables in the transmission grid. The only way to remedy this is to build compensation systems. However, these systems make grid operation more complex and make the transmission system more susceptible to faults. They take up a lot of space, cause noise – and are expensive. The cable study shows that building additional compensation systems for extensive underground cabling would cost around CHF 1,4 billion. These costs would have to be borne by electricity consumers.
Resonances: if oscillations are amplified, there is a risk of damage
Resonances are another complex physical phenomenon that influences reliable and stable grid operation. Every technical infrastructure has a natural frequency at which it «vibrates» by itself once it has been «activated». Even electrical grids have natural frequencies, called resonant frequencies. To prevent disturbances in the transmission grid, its resonant frequencies must be as far away as possible from any frequencies that can interfere with the lines. This includes the harmonics present in the grid, which are caused by the interaction of many different electrical non-linear consumers or power electronics such as rectifiers, frequency converters or motor controllers.
Grids with 380 or 220-kilovolt underground cables have a much lower resonant frequency than grids consisting exclusively of overhead lines due to the solid construction of the underground cables – like a tuning fork that «hums lower» the greater its mass. This leads to a growing risk that the resonant frequency of the underground cable grid and the interfering external influences will «amplify» each other. A similar effect was the undoing of the Broughton Suspension Bridge in northern England in 1831: the frequency of the steps of soldiers crossing the bridge in unison interfered with the natural frequency of the bridge, triggering oscillation amplification and ultimately causing the bridge to collapse.
The ability to reactivate lines after power system failures compromised
The lower resonant frequency of underground cable grids is a particular problem when lines are switched back on – for example after the rectification of disturbances or after planned shutdowns for maintenance work – because oscillation amplification can lead to damage or disconnections. An additional factor is that repairing an underground cable is significantly more complex and expensive than repairing an overhead line, given that underground cables are laid in the ground. Damaged underground cables therefore often remain out of service for weeks or even months.
An increase in the proportion of underground cables in the system as a whole can have a potentially dramatic effect in the event of power system failures. Power system failures are large-scale disturbances to the supply of electricity caused by the simultaneous outage of several grid elements in the transmission grid. In these – fortunately very rare – cases, it is vital for Switzerland to restore its grid as quickly and smoothly as possible. To do so, Swissgrid has divided the transmission grid into four grid restoration cells.
Each cell comprises an area with black start-capable power plants. After a power system failure, these power plants can use their own electricity generation to build up the necessary frequency, voltage and power to gradually restore the surrounding grids. The Swiss cable study shows that resonance effects and oscillation amplification can – depending on the length of the underground cables and their proximity to power plants with black-start capability – hinder or even completely prevent the ability of a cell to restore the grid after a large-scale power system failure.
Conclusion: continuing as before could jeopardise security of supply
The cable study shows that the installation of underground cables in the transmission grid is only possible to a very limited extent. It confirms the major technical and operational challenges associated with an increase in underground cables, which Swissgrid has been emphasising for some time. These challenges are based on the specific physical properties of underground cables. The study comes to the following conclusions:
- More systems, greater complexity and costs: underground cables generate much more reactive power than overhead lines. To avoid risky overvoltages and grid failures, this reactive power must be absorbed by additional compensation systems. Compensation systems increase the complexity of grid operations, take up space, generate noise and cause high costs.
- Difficult grid restoration after a power system failure: underground cables reduce the resonant frequencies in the transmission system. This increases the risk that the restoration of the grid may become impossible in entire regions after a power system failure due to oscillation amplification. In addition, resonance effects heighten the risk of instability in power plants and grid operations, as well as in grid components and electrical devices.
- The lower the proportion of cables, the more stable the grid operations: due to the phenomena demonstrated in the cable study, the proportion of underground cables in the transmission grid must be kept low – especially at the highest voltage level of 380 kilovolts.
Uncontrolled cabling on a «first come, first served» basis has a negative impact on grid stability and security of supply in Switzerland. In consultation with the authorities, Swissgrid is therefore endeavouring to develop a system that will make it possible to weigh up from a general perspective where underground cabling is a necessary and acceptable option for future grid projects.
Efficient approval processes for grid expansion thanks to a clear technology decision
The Swiss cable study confirms the principle put forward in the consultation draft of the Electricity Act for the conversion and expansion of the electricity grid («Grid express»): overhead lines should be prioritised in the transmission grid, while the possibility of laying underground cables should only be examined if certain criteria are met. This streamlines processes, reduces delays and will help to adapt the grid more quickly to the requirements of the energy transition. This will keep the grid stable and secure without burdening electricity consumers with significant additional costs.
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